1. Field of the Invention
The present invention relates to a power converter for powering a load from an AC source and for improving the power factor of the load.
2. Discussion of the Related Art
Electronic ballasts are frequently used instead of magnetic ballasts to control nonlinear loads such as fluorescent lamps. Compared to magnetic ballasts, electronic ballasts are smaller and lighter, do not have the flicker associated with 60 Hz power mains, and are more efficient. Electronic ballasts are especially desirable for powering the compact, consumer fluorescent lamps that are becoming more popular.
Most compact electronic ballasts consist of discrete components, including power transistors and pulse transformers. FIG. 1 shows a typical electronic ballast. As shown, the electronic ballast includes a half bridge totem pole driver formed from two discrete power MOSFET or bipolar transistors. The half bridge output drives a resonant load by means of feedback from a pulse transformer T1 whose primary winding is connected in series with the load. Each of two secondary windings of the transformer is connected to the input terminal of the half bridge transistors such that the load is driven synchronously. The LC resonating elements provide substantially sinusoidally varying voltage and/or current waveforms Since the switching frequency at the load is higher than the line frequency, small reactive components may be used, reducing the bulk and size of the ballast.
A disadvantage of the circuit of FIG. 1 is the operation of the circuit at low power factor, i.e., the current drawn from the power source is out-of-phase (usually leading) with the power source voltage, e.g., at about 50 to 60% lead. The low power factor is largely due to the circuit of FIG. 1 being configured with a peak detector rectifier and that the only time power is drawn from the source is when the source voltage is higher than the load voltage.
The power converter according to the present invention avoids the above disadvantage and operates with high power factor.